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General pedagogical ideas around optimal difficulty

Many theories of instruction suggest that learning is optimal when an appropriate level of challenge is maintained. If a task is too easy, there's little to learn. If a task is too difficult, the learner can be overwhelmed. The implication for practice is that task difficulty should increase in conjunction with the increased skill of the learner. Of course, we can think of exceptions to these ideas, such as the value of over-learning, and the general idea of exposing people to challenging problems to motivate further learning. This also still leaves open the question of precisely how fast difficulty should increase for a given learner.

Academically, I've seen these ideas expressed in several places:

Deliberate practice: This is a concept drawn from the expertise literature. It is posited that amount of deliberate practice (rather than amount of simple experience) is one of the key factors that discriminates experts from non-experts (see Ericsson et al 2006).
A key aspect of task difficulty relates to the sequencing of task difficulty with practice.
Ericsson et al (2006) writes that

The core assumption of deliberate practice ... is that expert
performance is acquired gradually and that effective improvement of
performance requires the opportunity to find suitable training tasks
that the performer can master sequentially... Deliberate practice
presents performers with tasks that are initially outside their
current realm of reliable performance, yet can be mastered within
hours of practice by concentrating on critical aspects and by
gradually refining performance through repetitions after feedback.

Challenge point in motor learning:
Guadagnoli and Lee present a framework for thinking about optimal difficulty in motor learning. Their framework suggests that optimal learning is achieved by maintaining task difficulty at the point where it provides optimal information (where learning is possible, but it is not too difficult as to be overwhelming).

Research on task difficulty and perceptual learning

In a review article Ahissar and Hochstein (2004) also suggest easy to difficult is the most effective regime for perceptual learning:

As found in our studies, training is more effective if subjects start
with easy conditions and gradually move to more difficult con-
ditions. The importance of beginning training with easy conditions was
first found by Pavlov. When Pavlov reinforced a dog’s salivation
following its seeing an ellipse but not following its seeing a circle
(Figure I, rightmost pair of stimuli), the dog could not avoid
generalization and salivated at sight of the circle, as well. Only by
using very elongated ellipses, and training along the continuum, from
left to right, was it able to achieve good performance for small
circle/ellipse differences. This phenomenon was subsequently termed
‘transfer along a continuum’ (of different degrees of difficulty).

References

Ericsson, K. et al. (2006). The influence of experience and deliberate
practice on the development of superior expert performance. The
Cambridge handbook of expertise and expert performance, 10(3):683-703.
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Thanks for the detailed answer. BTW: Merav Ahissar is my PhD advisor :) ... What motivated this question was my feeling that some of these claims are based on 'experimenters experience' rather than on actual rigorous testing of the matter. I'll check out the other leads.
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Ofri RavivFeb 3 '12 at 6:36

Wow. That's a small world. I think the optimal pacing of difficulty with practice is an interesting question, and despite the support of many broad theories, many interesting questions remain when it comes to the exact levels of difficulty on a specific task.
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Jeromy Anglim♦Feb 3 '12 at 6:39